Functionalized Nanotube Material for Supercapacitor Electrodes

ABSTRACT

The present teachings are directed towards supercapacitors including an electrode composed of a functionalized carbon nanotube-containing material, and also an organic electrolyte solution. The organic electrolyte solution can contain a non-aqueous solvent and an ionic salt, examples of non-aqueous solvent include propylene carbonate, ethylene carbonate, sulfolane and acetonitrile. The functional groups present on the carbon nanotubes can include hydroxyl groups, carboxyl groups, alkoxyl groups and mixtures thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 11/248,299 filed Oct. 13, 2005, which is now U.S.Pat. No. ______, which is incorporated herein in its entirety byreference for all purposes.

BACKGROUND

1. Field of the Invention

This invention relates to functionalized carbon nanotubes and materialcontaining functionalized carbon nanotubes suitable for use aselectrodes and as electrode material in supercapacitors.

2. Discussion of the Related Art

It is well known in the art that carbon nanotubes can exhibitsemiconducting or metallic behavior. Additional properties that makecarbon nanotubes of interest are high surface area, high electricalconductivity, high thermal conductivity and stability, and goodmechanical properties. See U.S. Patent Application Publication US2003/0164427 A1.

Supercapacitors typically have specific capacitance of greater than 100F/g, as opposed to conventional capacitors with specific capacitance onthe order of only several F/g. Supercapacitors refer to, withoutlimitation, electrochemical capacitors, electric double layer capacitorsand ultracapacitors.

Capacitor electrodes prepared from carbon nanotube-containing materialscan exhibit high levels of performance. However, for supercapacitorapplications even higher levels of capacitance performance are desirableor required.

A need exists, therefore, for nanotube-containing materials for use inas electrodes in supercapacitors, or capacitors, exhibiting higherlevels of capacitance.

SUMMARY

The present teachings meet the needs for supercapacitor electrodematerials exhibiting higher capacitance levels by providing compositionsof functionalized carbon nanotube-containing materials suitable forelectrodes for supercapacitors.

The present teachings include a supercapacitor having one or moreelectrodes made of a functionalized carbon nanotube-containing material,and an organic electrolyte solution. Further taught is a supercapacitorelectrode having a functionalized carbon nanotube-containing materialsubstantially free of binder material.

A method of producing electrodes by oxidizing carbon nanotube-containingmaterial, contacting the oxidized carbon nanotube-containing materialwith functional groups to form functionalized carbon nanotube-containingmaterial, and preparing electrodes from the functionalized carbonnanotube-containing material is also provided in the present teachings.The oxidizing and contacting steps can be one step, or separate methodsteps.

The present teachings also include a method of producing supercapacitorelectrodes by contacting carbon nanotubes with an oxidizer to formoxidized carbon nanotubes. Functional groups are then added to theoxidized carbon nanotubes to form functionalized carbon nanotubes whichcan then be formed into a film. The film can then be prepared into asupercapacitor electrode containing the functionalized carbon nanotubefilm.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is included to provide a furtherunderstanding of the present teachings and is incorporated in andconstitute a part of this specification, illustrate various embodimentsof the present teachings and together with the detailed descriptionserve to explain the principles of the present teachings. In thedrawing:

FIG. 1 is a TEM micrograph of purified HiPCO single-walled carbonnanotubes; and

FIG. 2 is a plot of current versus specific capacitance per mass forsupercapacitor electrodes containing carbon nanotube-containing materialtreated with various concentrations of nitric acid.

DETAILED DESCRIPTION

The present teachings are directed to materials composed offunctionalized carbon nanotubes, methods of preparing those materials,use of those materials as electrodes in supercapacitors, andsupercapacitors including functionalized carbon nanotube-containingelectrodes.

A supercapacitor having one or more electrodes including afunctionalized carbon nanotube-containing material, and an organicelectrolyte solution is provided by the present teachings. Thefunctionalized carbon nanotube-containing material can be substantiallyfree of any binder material. Binder material, such as, for example,polytetrafluoroethylene (PTFE), are typically added to carbonnanotube-containing materials to facilitate the formation of pellets orfilms.

The organic electrolyte solution includes a non-aqueous solvent and anionic salt. The non-aqueous solvent can include at least one memberselected from the group consisting of propylene carbonate, ethylenecarbonate, sulfolane and acetonitrile. The ionic salt can include atleast one member selected from the group consisting oftriethylmethylammonium tetrafluoroborate, triethylmethylammoniumhexafluorophosphate, tetraethylammonium tetrafluoroborate,tetraethylammonium hexafluorophosphate, tetrabutylammoniumtetrafluoroborate and tetrabutylammonium hexafluorophosphate. One ofskill in the art will recognize the numerous possible variations in thecombination of alkyl groups in the ammonium salt. One non-aqueoussolvent and ionic salt combination of particular interest is propylenecarbonate and triethylmethylammonium tetrafluoroborate.

A supercapacitor according to the present teachings can include anelectrolyte solution that can sustain voltages of greater thanplus/minus about 1.25 V, or greater than plus/minus about 1.5 V, orgreater than plus/minus 2.0 V. The organic electrolyte solutions setforth above can be utilized for some of the various embodiments of thepresent teachings.

The functionalized carbon nanotube-containing material can includecarbon nanotubes having functional groups attached thereto. Thosefunctional groups can be attached to the unfunctionalized carbonnanotubes during oxidation of the unfunctionalized carbon nanotubes.

According to the present teachings, carbon nanotubes functionalized asset forth herein can exhibit increased aqueous wettability propertiesand dispersability in aqueous solutions over similar unfunctionalizedcarbon nanotubes. Dispersability may also be referred to as the apparentsolubility of the functionalized carbon nanotubes in aqueous solutions.These soluble functionalized carbon nanotubes can be used for preparingconducting polymer-carbon nanotube composites, for example, compositesprepared from, for instance, PEDOT or polyaniline added to the solublefunctionalized carbon nanotubes. Additionally, further enhancements insupercapacitance can be obtained by the chemically or electrochemicallypolymerization of conducting polymers on or around the solublefunctionalized carbon nanotubes.

The unfunctionalized carbon nanotubes utilized can be nanotubes thatinclude single-walled carbon nanotubes and multiple-walled carbonnanotubes. Of particular interest are single-walled nanotubes. Thecarbon nanotubes can have an outer diameter ranging from about 0.5 nm toabout 1 nm, or ranging from about 1 nm to about 10 nm, or ranging fromabout 10 nm to about 25 nm, or ranging from about 25 nm to about 45 nm,or ranging from about 45 nm to about 100 nm. The carbon nanotubes canhave average bundle sizes ranging from about 20 nm to about 30 nm. Thecarbon nanotubes can have average lengths ranging from about 50 nm toabout 1 mm.

Suitable nanotubes can be formed by any suitable method, for example,laser ablation of carbon, decomposition of a hydrocarbon, or arcingbetween two carbon graphite electrodes. Numerous references describesuitable methods and starting materials to produce suitable carbonnanotubes. See, for example, U.S. Pat. Nos. 5,424,054; 6,221,330 and6,835,366; Smalley, R. E., et al., Chem. Phys. Lett. 243, pp. 1-12(1995); and Smalley, R. E., et al., Science, 273, pp. 483-487 (1996).Suitable carbon nanotubes are commercially available from a number ofsources. Single-walled nanotubes (herein referred to as “SWNT”) areavailable from Carbon Nanotechnologies, Inc. of Houston, Tex.

The functional groups added to the unfunctionalized carbon nanotubes caninclude at least one member selected from the group consisting ofhydroxyl groups, carboxyl groups, alkoxyl groups and mixtures thereof.Of particular interest are hydroxyl and carboxyl groups. The extent offunctionalization can be measured by observing the appropriate hydroxyland/or carboxyl bands of the IR spectrum.

The functionalized carbon nanotube-containing material can be in theform of a film. The film can be produced by heating a dispersion of thefunctionalized carbon nanotube-containing material to a temperaturesufficient to produce a film, for instance, a temperature ranging fromabout 90 C. to about 110 C. The removal of water or solvent from thedispersion of functionalized carbon nanotube-containing material to forma film can be facilitated by use of a PTFE-coated evaporation vessel.

Another embodiment of the present teachings includes a capacitorelectrode containing a functionalized carbon nanotube-containingmaterial substantially free of binder material. The functionalizedcarbon nanotube-containing material can include carbon nanotubes havingfunctional groups attached thereto during oxidation of the carbonnanotubes.

Yet another embodiment of the present teachings includes a method ofproducing electrodes which can include oxidizing carbonnanotube-containing material, contacting the oxidized carbonnanotube-containing material with functional groups to formfunctionalized carbon nanotube-containing material, and preparingelectrodes from the functionalized carbon nanotube-containing material.

The method can further include forming a film of the functionalizedcarbon nanotube-containing material.

According to the method of producing electrodes, the oxidizing step canbe contacting the carbon nanotube-containing material with an oxidizer.Suitable oxidizers can be at least one member selected from the groupconsisting of bromates, chlorates, chromates, iodates, nitrates,nitrites, perborates, percarbonates, perchlorates, periodates,permanganates, peroxides and persulfates. Another suitable oxidizer isnitric acid.

Additionally, in some embodiments of the present teachings, thecontacting step can occur at the same time as the oxidizing step, orcontacting the carbon nanotubes with the functional groups can be aseparate process step.

The functional groups suitable for the present method include at leastone member selected from the group consisting of hydroxyl group,carboxyl group, alkoxy group and mixtures thereof.

The film forming step can include heating the functionalized carbonnanotube-containing material to a temperature sufficient to form a film.An example of a suitable sufficient temperature for forming a film caninclude a temperature ranging between about 90 C and about 110 C.

The carbon nanotube-containing material suitable for the present methodincludes single-walled carbon nanotubes. Such single-walled carbonnanotubes can have average bundle sizes ranging from about 20 nm toabout 30 nm, and can have average lengths ranging from about 50 nm toabout 1 mm.

Another embodiment of the present teachings includes a method ofproducing supercapacitor electrodes including contacting carbonnanotubes with an oxidizer to form oxidized carbon nanotubes, and addingfunctional groups to the oxidized carbon nanotubes to formfunctionalized carbon nanotubes. The functionalized carbon nanotubes canthen be formed into a film, and a supercapacitor electrode can beprepared from the functionalized carbon nanotube film.

The oxidizer utilized in the present method can include at least onemember selected from the group consisting of bromates, chlorates,chromates, iodates, nitrates, nitrites, perborates, percarbonates,perchlorates, periodates, permanganates, peroxides and persulfates. Ofparticular interest is use of nitric acid as the oxidizer of the presentmethod.

The functional groups utilized in the present method can include atleast one member selected from the group consisting of hydroxyl group,carboxyl group, alkoxy group and mixtures thereof. Hydroxyl and carboxylgroups are of particular interest.

The film forming step of the present method can be accomplished byheating the functionalized carbon nanotubes to a temperature sufficientto form a film, such as a temperature ranging between about 90 C. andabout 110 C.

The carbon nanotube-containing material suitable for the present methodscan include single-walled carbon nanotubes. The single-walled carbonnanotubes used in the method can have average bundle sizes ranging fromabout 20 nm to about 30 nm, and can have average lengths ranging fromabout 50 nm to about 1 mm.

As used herein, a reference electrode refers to a standard silver/silverchloride/sodium chloride electrode as known to those in the art.

All publications, articles, papers, patents, patent publications, andother references cited herein are hereby incorporated herein in theirentireties for all purposes.

Although the foregoing description is directed to the preferredembodiments of the present teachings, it is noted that other variationsand modifications will be apparent to those skilled in the art, andwhich may be made without departing from the spirit or scope of thepresent teachings.

The following examples are presented to provide a more completeunderstanding of the present teachings. The specific techniques,conditions, materials, and reported data set forth to illustrate theprinciples of the principles of the present teachings are exemplary andshould not be construed as limiting the scope of the present teachings.

EXAMPLES Example 1

A sample of HiPCO carbon nanotubes (obtained from CarbonNanotechnologies, Inc. of Houston, Tex.) was heated in air at 325 C forninety minutes. After the heat treatment, the cooled carbon nanotubeswere stirred in concentrated HCl for 14 hours. The carbon nanotubes werefiltered out of the acid solution and washed well with deionized water.The washed carbon nanotubes were dried at 110 C for 3 hours, and thenheated to 150 C in a 10⁻³ torr vacuum for 12 hours.

A TEM micrograph of the treated carbon nanotubes is presented in FIG. 1.Average bundle size of the treated carbon nanotubes is between about 20and about 30 nm.

Example 2

Nitric acid was used in a range of concentrations to analyze the acidconcentration effect on the functionalized carbon nanotube material.

Four different samples of the carbon nanotubes prepared in Example 1above were separately refluxed at 100 C for 24 hours in different molarconcentrations of nitric acid, except for the concentrated nitric acidsample which was refluxed for one hour. The nitric acid concentrationswere 3.0 M, 6.0 M, 10.0 M, and concentrated HNO₃ (70% by volume). Thenanotubes were then filtered off, washed with deionized water, and thencentrifuged at 5000 rpm for 15 minutes to remove insoluble nanotubes.

Aqueous dispersions of the soluble nanotubes were then cast inPTFE-coated evaporation vessels, heated in a 100 C oven with a gentleflow of air for 12 hours to remove water, and formed into films withapproximate thicknesses of about 20 to about 30 microns.

Each film was then adhered to an aluminum electrode using a colloidalgraphite paste and dried at 150 C in a 10⁻³ torr vacuum for at least 12hours. The aluminum electrodes were prepared by wiping with ethanol andacetone, etching with 0.2 N NaOH solution for five minutes, washing withdeionized water, and then drying overnight at 90 C in flowing air beforeadhering the film.

An additional electrode was prepared using carbon nanotube as preparedin Example 1 above without further treatment and 10 wt. %polytetrafluoroethylene (PTFE) as a binder. The nanotube/binder mixturewas mixed with a mortar and pestle, and then hydraulically pressed intoa pellet with a nominal thickness of about 20 to about 30 microns.

The cyclic voltammetric test cell utilized a standard Ag/AgCl referenceelectrode, a platinized Pt counter electrode, with the samples attachedto an aluminum strip electrode using colloidal graphite paste. The testcell was purged with a flow of argon gas. The electrolyte solution was0.1 M triethylmethylammonium tetrafluoroborate in propylene carbonate.

The samples were electrochemically activated by cycling from 0 to −2 Vat 1 mV/s for two cycles. The capacitance of all five samples was thenmeasured by cycling between about +2.0 V and about −2.0 V at a scan rateof 1 mV/s, with the capacitance calculated from the second cyclemeasurements. The results are presented in the graph of FIG. 2.

The nanotube yield, solubility in water and the intensity of thecarboxyl cm⁻¹) and hydroxyl (3450 cm⁻¹) peak heights were also analyzed.The results are presented below in Table 1.

TABLE 1 Yield of Soluble Solubility 1735 cm⁻¹ 3450 cm⁻¹ Nanotubes (mg/mLpeak height peak height Sample (%) H₂O) (arb units) (arb units) Example1 n/a 0.0 0.0 0.0 3 M Nitric Acid 70.5 0.75 0.06 0.25 6 M Nitric Acid68.3 0.88 0.09 0.35 10 M Nitric Acid  60.3 1.10 0.115 0.50 Conc. NitricAcid 21.2 1.24 0.135 0.65

The foregoing detailed description of the various embodiments of thepresent teachings has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit the presentteachings to the precise embodiments disclosed. Many modifications andvariations will be apparent to practitioners skilled in this art. Theembodiments were chosen and described in order to best explain theprinciples of the present teachings and their practical application,thereby enabling others skilled in the art to understand the presentteachings for various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the present teachings be defined by the following claims and theirequivalents.

1. A supercapacitor comprising one or more electrodes comprising a functionalized carbon nanotube-containing material, and an organic electrolyte solution.
 2. The supercapacitor according to claim 1, wherein the organic electrolyte solution comprises a non-aqueous solvent and an ionic salt.
 3. The supercapacitor according to claim 2, wherein the non-aqueous solvent comprises at least one member selected from the group consisting of propylene carbonate, ethylene carbonate, sulfolane and acetonitrile.
 4. The supercapacitor according to claim 2, wherein the ionic salt comprises at least one member selected from the group consisting of triethylmethylammonium tetrafluoroborate, triethylmethylammonium hexafluorophosphate, tetraethylammonium tetrafluoroborate, tetraethylammonium hexafluorophosphate, tetrabutylammonium tetrafluoroborate, and tetrabutylammonium hexafluorophosphate.
 5. The supercapacitor according to claim 1, wherein the functionalized carbon nanotube-containing material comprises carbon nanotubes having functional groups attached thereto.
 6. The supercapacitor according to claim 5, wherein the carbon nanotubes are functionalized during oxidation.
 7. The supercapacitor according to claim 5, wherein the carbon nanotubes comprise single-walled carbon nanotubes.
 8. The supercapacitor according to claim 7, wherein the single-walled carbon nanotubes have average bundle sizes ranging from about 20 nm to about 30 nm.
 9. The supercapacitor according to claim 7, wherein the single-walled carbon nanotubes have average lengths ranging from about 50 nm to about 1 mm.
 10. The supercapacitor according to claim 5, wherein the functional groups comprise at least one member selected from the group consisting of hydroxyl groups, carboxyl groups, alkoxyl groups and mixtures thereof.
 11. The supercapacitor according to claim 1, wherein the functionalized carbon nanotube-containing material comprises a film.
 12. The supercapacitor according to claim 1, wherein the functionalized carbon nanotube-containing material is substantially free of binder material.
 13. A capacitor electrode comprising: a functionalized carbon nanotube-containing material.
 14. The capacitor electrode according to claim 13, wherein the functionalized carbon nanotube-containing material comprises carbon nanotubes having functional groups attached thereto.
 15. The capacitor electrode according to claim 14, wherein the carbon nanotubes are functionalized during oxidation.
 16. The capacitor electrode according to claim 14, wherein the carbon nanotubes comprise single-walled carbon nanotubes.
 17. The capacitor electrode according to claim 16, wherein the single-walled carbon nanotubes have average bundle sizes ranging from about 20 nm to about 30 nm.
 18. The capacitor electrode according to claim 16, wherein the single-walled carbon nanotubes have average lengths ranging from about 50 nm to about 1 mm.
 19. The capacitor electrode according to claim 16, wherein the functional groups comprise at least one member selected from the group consisting of hydroxyl groups, carboxyl groups, alkoxyl groups and mixtures thereof.
 20. The capacitor electrode according to claim 13, wherein the functionalized carbon nanotube-containing material comprises a film.
 21. The capacitor electrode according to claim 13, wherein the functionalized carbon nanotube-containing material is substantially free of binder material. 